How energy-positive is incinerating the plastic when it needs dedicated collection, sorting, processing into pellets, and specialized incineration to scrub waste gasses, compared to collecting natural gas and burning it?

The difference is what determines whether burying or burning the plastic is better from a C02 emission standpoint.

Also, if we don't have enough plastic to completely offset the need for natural gas energy production, then the biggest cost of natural gas- the drilling and transporting- isn't a part of the equation.