Numerical analysis and modelling of gas flow within MSW landfill has the potential to significantly enhance the efficiency of design and performance of operation for landfill gas extraction and utilization projects. Available models implement a simple theoretical basis and treat the landfilled waste fill purely as a porous medium with the conservation of mass inherently assumed and the ongoing gas generation thus ignored. The research described in this work was intended to evaluate the error introduced into estimates of the intrinsic permeability of waste from landfill gas pumping tests if the ongoing gas generation within the landfilled MSW is ignored when the pumping tests are evaluated to yield estimates of the permeability. Two different approaches were used to simulate gas flow in the unsaturated waste fill: GeoStudio2007 software suite and a 1-D FD solution. This work demonstrated that the gas generation term should not be ignored when the gas flow is evaluated for MSW. Thus, the correction charts is developed as a first step toward a reliable method that would enable such widely used software to be used with a correction factor to enable improved simulations of gas flow.