Catalysis of the reaction: coenzyme B + methyl-coenzyme M = coenzyme M-coenzyme B heterodisulfide + methane. Methyl-CoM is also known as 2-(methylthio)ethanesulfonate, coenzyme B as N-(7-mercaptoheptanoyl)threonine 3-O-phosphate, and coenzyme M-coenzyme B heterodisulfide as CoM-S-S-CoB.