Methods for utilizing a supercritical water unit to convert waste plastics to product through hydrothermal treatment in a supercritical unit are provided. Waste plastic is treated in a pretreatment unit, melting the plastic into a liquid and prepares the plastic for the supercritical water unit. The pretreatment unit can dehalogenate the waste plastic. The molten plastic is introduced into a supercritical water unit with water, which generates a product. A flushing stream of product and steam or water from the supercritical water unit is recycled from the supercritical water unit into the pretreatment unit, preheating and pretreating the waste plastic, and acting as a catalyst in the dechlorination reaction. A purge stream removes the products of the dehalogentation reaction occurring in the melting section.