1) You already have oil-powered power plants, and
2) You have efficient, modern trash incinerators that can recover 90%+ of the energy in plastics, and
3) You make sure that all plastic garbage actually ends up incinerated.
Because the utility you get out of making plastics out of oil, using the plastics for something, and then burning it, is higher than just straight up burning the oil.
But if you fail any of these three points, you're better off banning single-use plastics.
Oddly, this seems to be the most difficult part of the equation here in NYC. Containerization needs to be adopted in order to make the system more efficient and sanitary. Containerization would prevent refuse from spilling into the street, into the storm drains, etc. The current system (curbside pickup of plastic bags) falls over (literally and figuratively) in too many ways and the sanitation workers aren't interested in picking up the slack -- they ignore anything not secured in a plastic bag.
Zurich has a very centrally located incinerator (behind the "viadukt" shopping area) and I believe that they generate electricity from the plant. They also have a very granular sorting regime ... it would appear they are covering all three of the bases.
- consumer-sorted recycling is a BandAid virtue-signal and ineffective because it gets mixed up, under-sorted and contaminated
- post-consumer waste-stream separation works. Stanford does it because five bins was ridiculous
- all trash should be recycled, composted at-scale to reach near zero landfill stream
- Structural waste reduction by source-return reuse, recycling
- single-use plastics should be eliminated
- plastics that cannot be recycled should be banned from retail sale, packaging, products and landfills
Maybe very large taxes on landfill waste would compel recycling and manufacturing to get their acts together?