If you comparing making electricity from "clean burning" NG vs from coal,
it turns out that Methane (NG) is AT LEAST 1.2x (20%) worse than coal
and may be up to 2x (100%) worse at the 20 year time horizon. (Howarth),
This is because NG production and transmission is a notoriously leaky process.
Not all methane is burned. Lots is lost due to various
vectors for fugitive emissions (venting though faults, well casing failures,
pipeline leaks, blowdowns, etc.).
Methane, a potent greenhouse
gas, is > 20 times more powerful than
carbon dioxide at trapping heat. (Steingraber)
Considering all factors, raw methane when leaked into the atmosphere
is 105 times greater impacts of that of CO2.