[Manuel_D]

Yes, but it'd be net-zero based on the carbon used to produce the fuel. In theory at least, in practice most synthetic methane has only been produced by scavenging CO2 byproducts from the chemical industry. It's not truly net-zero rather it's releasing CO2 that would have been emitted anyway. CO2 is in too small concentrations in the atmosphere to effectively capture.

[sanderjd]

What I was missing was that the assumption here is that the input carbon is being sucked out of the air. As you point out, that isn't the only way to get it...