WordNet (r) 3.0 (2006):

tie in
    v 1: be in connection with something relevant; "This ties in
         closely with his earlier remarks"
    2: make a logical or causal connection; "I cannot connect these
       two pieces of evidence in my mind"; "colligate these facts";
       "I cannot relate these events at all" [syn: associate, tie
       in, relate, link, colligate, link up, connect]
       [ant: decouple, dissociate]

Moby Thesaurus II by Grady Ward, 1.0:

225 Moby Thesaurus words for "tie in":
   accord, act in concert, act together, addition, adjunct, affairs,
   affect, affiliate, affiliation, affinity, agglomeration,
   agglutination, aggregation, alignment, alliance, ally, amalgamate,
   amalgamation, answer to, appertain to, apply to, approximation,
   articulation, assemblage, associate, association, band,
   band together, be in cahoots, be in league, bear on, bear upon,
   belong to, bill of sale, bond, bracketing, bunch, bunch up, cabal,
   cahoots, cement a union, centralize, closeness, club,
   club together, clustering, coact, coadunation, coalesce,
   coalescence, coalition, collaborate, colleagueship, collegialism,
   collegiality, collude, combination, combine, come together,
   communication, comradeship, concatenation, concern, concert,
   concord, concourse, concur, concurrence, confederacy, confederate,
   confederation, confluence, confraternity, congeries,
   conglomeration, conjugation, conjunction, connect, connectedness,
   connection, consociate, consolidate, consolidation, conspire,
   contiguity, contrariety, convergence, cooperate, copartnership,
   copartnery, copulation, correspond to, couple, coupling, deal with,
   dealings, deduction, demand, disjunction, do business with,
   federalize, federate, federation, fellowship, filiation,
   fraternalism, fraternity, fraternization, freemasonry, fuse,
   fusion, gang, gang up, gathering, get heads together, get together,
   go in partners, go in partnership, go partners, hang together,
   harmonize, have connection with, hold together, homology, hook up,
   hook up with, hookup, inclusion, incorporation, integration,
   intercommunication, intercourse, interest, interlinking, intimacy,
   involve, join, join forces, join fortunes with, join in,
   join together, join up with, join with, joinder, joining, jointure,
   junction, keep together, knotting, league, league together,
   liaise with, liaison, link, link with, linkage, linking,
   make common cause, market, marriage, marry, mass market, meeting,
   merge, merger, merging, mutual attraction, nearness, organize,
   outlet, pair, pair off, pairing, partner, partnership, pertain to,
   play ball, propinquity, proximity, pull together,
   put heads together, rapport, reciprocate, refer to, regard,
   relate to, relatedness, relation, relations, relationship, respect,
   retail, sale, similarity, sodality, sorority, splice,
   stand together, stand up with, symbiosis, sympathy, team up,
   team up with, team with, throw in together, throw in with, tie,
   tie in with, tie up, tie up with, tie-up, touch, touch upon,
   treat of, turnover, unification, union, unionize, unite,
   unite efforts, unite with, wed, wholesale, work together, yoking