There's more to a flatus (dare I say fart?) than just methane gas. Firstly and I think most obviously (yet we are loathe to admit it) is that the gases do pick up various things on its way out. It has no choice since its going down the path. However, most of the smells are combinations of mercaptans and sulfides (read: sulfur). I do believe there are other gases involved as well.
Anyway, it couldn't be just one gas because as we all can attest, farts smell different from person to person and even from meal to meal within individuals. Basically, how a fart will smell will depend on the meals eaten. Beans produce high volumes of gas but provide little stench, meats and eggs on the other hand seem to produce vile smelling gas.
Without becoming disgusting, I'd imagine that particulate matter in various forms is picked up and expelled, but (no pun intended) I suspect that your cotton (you do wear cotton don't you?) underwear will trap that up. But I cannot say this with absolute authority.
The moral of the story? Don't eat steak and eggs over a bed of beans.