Plastic is a polymer. Yes, it can be converted by depolymerization. Depolymerization (or depolymerisation) is the process of converting a polymer into a monomer or a mixture of monomers. So if your monomer is gas, you solid plastic be can converted to gas. Most polymers can be depolymerizated by photolysis to give lower molecular weight molecules. Electromagnetic waves with the energy of visible light or higher, such as ultraviolet light, X-rays and gamma rays are usually involved in such reactions.