[pfdietz]

3% of Natural Gas, not methane. Natural gas contains other gases, in particular ethane, which is the feedstock for making ethylene and then polyethylene.

US fracked gas is quite high in ethane.

Some of the natural gas used for plastics is to make heat, for example to drive ethylene crackers (which decompose ethane at high temperature). Methane could be burned for that.

There was a process for oxidative coupling of methane to make ethylene (a Bay Area startup named Siluria created it, using a cool phage-based combinatorial method to find structured catalysts) but I don't think it could compete with all that cheap natural ethane.