The Collaborative International Dictionary of English v.0.48:

Synthesize \Syn"the*size\, v. t.
   1. To combine by synthesis; to unite.
      [1913 Webster]

   2. To produce by synthesis; as, to synthesize albumin.
      [1913 Webster] Synthetic

WordNet (r) 3.0 (2006):

synthesize
    v 1: combine so as to form a more complex, product; "his operas
         synthesize music and drama in perfect harmony"; "The liver
         synthesizes vitamins" [syn: synthesize, synthesise]
         [ant: analyse, analyze, break down, dissect, take
         apart]
    2: combine and form a synthesis; "Vitamin D only synthesizes
       when sunlight is available"

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "synthesize":
   add, amalgamate, analyze, apply reason, arrange, assemble,
   assimilate, blend, build, build up, coalesce, combine,
   come together, compose, compound, comprise, connect, consist of,
   consolidate, constitute, construct, deduce, embody, encompass,
   enter into, fabricate, flux, form, fuse, generalize, go into,
   hypothesize, identify, include, incorporate, infer, integrate,
   intellectualize, interblend, interfuse, join, logicalize, logicize,
   lump together, made, make, make one, make up, meld, melt into one,
   merge, merge in, mix, orchestrate, organize, philosophize,
   piece together, provide a rationale, put together, rationalize,
   reason, reembody, roll into one, shade into, solidify, structure,
   syllogize, symphonize, syncretize, syndicate, theorize, unify,
   unite, unite in, use reason