I am a chemist, not a plumber. You are NOT smelling methane, methane is a colorless, odorless gas that is used as a fuel. When people say they smell “gas” they are actually smelling ethanthiol, an additive to give home gas its charectoristic odor. If it smells like rotten eggs, that is most likely hydrogen sulfide (HS2) which is dangerous. It is a waste product of different bacteria. You may want to run water down the drains if they havnt been used in awhile. If that doesn’t work, pour bleach down each drain and do NOT use them for a few hours. bleach contains sodium hypochlorite, which is a strong biocide. that would kill whatever bacteria is releasing that odor. good luck, and let me know how it goes.
Another solution to this smell could be to pour baking soda and vinegar into the drains, let it sit a few hours or overnight. Good Luck, Bill The Local Plumber Tustin, California http://www.TheLocalPlumber.com