The Collaborative International Dictionary of English v.0.48:

methanogen \methanogen\ n.
   A type of archaebacteria found in anaerobic environments such
   as animal intestinal tracts or sediments or sewage, and
   capable of producing methane; a source of natural gas.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

methanogen
    n 1: archaebacteria found in anaerobic environments such as
         animal intestinal tracts or sediments or sewage and capable
         of producing methane; a source of natural gas