Ignoring the positive externalities, there is still some environment cost when using synthetic CH4. You're still taking water from the ground and moving it into

Ignoring the positive externalities, there is still some environment cost when using synthetic CH4. You're still taking water from the ground and moving it into the upper atmosphere (CO2 + ground H2O -> CH4 + O2 -> CO2 + atmospheric H2O) - which contributes to the greenhouse effect. It's not worth worrying about, given all the other more impactful ways we can move the needle on global warming, but it's not actually zero.