To develop a predictive modeling capability for municipal waste gasification process, a 3-dimensional simulation code is developed based on the SIMPLE algorithm with momentum interpolation for pressure-correction. The two-phase flow and heat transfer processes due to the solid waste stack is modeled by a single-equation porous medium formulation with Darcy approximation. The pyrolysis process is described by a nonlinear Eulerian pyrolysis model to determine the porosity profile as well as the corresponding pyrolysis rate of the waste material. This generalized method simplifies the multiphase governing equations into gas phase equations. Polymethyl methacrylate (PMMA) was used as a test waste material for the simulation. Combustion behavior corresponding to the waste porosity, inlet conditions is studied for a range of operating conditions.