Clark,
essentially Methane is inexhaustible, for one thing, Methane is made by Methanogens, constantly, it is always being renewed but even if we wish to assume most Methane as fossil, there is a stunning amount of Clathrate, in Russia, Canada and most probably in the Antarctica.
Also we can encourage Methane by biogenesis. This has been done on farms for some time.
Farms can produce their own Methane, then power their plant with their home produced Methane, cuts down on shipping it over from America.
However, we only have to use Methane in the near term, until all our electricity is produced renewably.
A question, what else could so easily provide base line electricity?