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The main purpose of this study is to improve the numerical evaluation of gas injection enhanced coalbed methane (ECBM) recovery by establishing a new mathematical coalbed methane (CBM) simulation model, in which the two-dimensional Peng–Robinson equation of state (2D PR EOS) model and the Maxwell-Stefan bidisperse pore diffusion (MS-BPD) model for multicomponent gas adsorption and diffusion are implemented. First, the extended Langmuir, ideal adsorbed solution and 2D PR-EOS models were investigated and compared with the experimental data of pure and mixed gas for modeling multicomponent gas adsorption behavior. Then, the MS-BPD model was developed based on the 2D PR-EOS model, bidisperse diffusion mechanisms, and Maxwell-Stefan diffusion theory, which provided an improved simulation of multicomponent gas diffusion behavior. On the basis of previous research, we developed a new CBM simulation model to analyze the effects of injection gas composition on the gas injection ECBM recovery process. The results indicate that mixed-gas injections are able to enhance methane recovery. The optimum composition of injected gas is 75% N 2 and 25% CO 2 , which increases methane recovery by 25% over the pressure depletion production process. This CBM simulation model will provide an effective technical approach for research and development of CBM by gas injection. [ABSTRACT FROM AUTHOR]