The energy recovery of plastic waste provides a sustainable solution to the growing accumulation of these materials in the environment. In this study, a pyrolysis-gasification model was developed using Aspen Plus to investigate the effect of gasification reactor temperature on the products generated from the thermal conversion of three waste types: polyethylene (PE), polyethylene terephthalate (PET), and used tires. The results show that increasing the temperature favors the formation of synthesis gas (H₂ and CO), while the production of liquids and methane (CH₄) significantly decreases beyond 800 °C. PE proves to be particularly efficient for hydrogen generation, whereas PET exhibits near-complete conversion to gas at elevated temperatures. Although ammonia (NH₃) production remains limited, it is affected by the nitrogen content of the feedstock—especially in tires—and by the presence of inert nitrogen (N₂), which may interact under extreme conditions. These findings can assist industrial operators in optimizing gasification conditions for syngas production, thereby contributing to waste-to-energy development and circular economy strategies.