Landfill methane must be captured to reduce emissions of greenhouse gases; moreover it can be used as an alternative energy source. However, despite the widespread use of landfill gas (LFG) collection systems for over three decades, little information about their capture efficiency is available, because LFG generation rates usually remain unknown. Therefore, to assess the efficiency of greenhouse gas capture and to estimate the amount of fugitive emissions, LFG generation rates should be properly determined. In addition, to improve the capture efficiency of methane while minimizing air intrusion from the atmosphere, it is important to quantify gas flow patterns within landfills. In this study, a methodology to quantify methane generation rates and to estimate the gas permeability field was examined using inverse modeling. To account for the heterogeneous, but spatially correlated structure of refuse, the pilot point method involving geostatistical techniques and optimization algorithms was used. Synthetic observation data were generated from forward simulations for a pumping test and a baro-pneumatic test, and these data were used to test the inversion procedure. The inverse model was able to reproduce the spatial permeability distribution using the transient pressure changes in response to the withdrawal of LFG during the pumping test. The LFG generation rate was also successfully estimated using the data from the baro-pneumatic test with errors less than 2%. While this methodology was developed and successfully tested using synthetic data, it will be investigated in the future using field data from the bioreactor test cells at the Yolo County Central Landfill, CA.