Polyethylene contributes most to post-consumer plastic waste worldwide and gasification is one of the thermo-chemical methods applied to plastic waste. In this study, the gasification of polyethylene waste was modeled using the Gibbs free energy minimization and Lagrange method of undetermined multipliers, and was then validated. A central composite design was employed to assess and optimize the polyethylene waste gasification. The findings revealed that hydrogen production was significantly improved by 48% by raising the steam to polyethylene waste ratio according to the water–gas shift and reforming reactions. Hydrogen production and exergy efficiencies were enhanced by increasing temperature. Regression models produced from the central composite design had high accuracy for estimating the heat required and exergy efficiency with R2 values of 91.6% and 96.8%, respectively. Multi-objective optimization was performed to minimize the heat required and maximize exergy efficiency. A steam to polyethylene waste ratio of 1 and temperature of 700 °C were the optimum conditions. Required heat of 213.7 kJ and exergy efficiency of 95.37% were the responses at the optimum state.
