An approximate two-stage kinetic model of the chemical reaction in methane/oxygen mixtures containing water and inert diluents is developed. The model includes one differential equation for the calculation of the molar mass of the gas after the induction period and algebraic formulas for the calculation of the heat release, internal energy, and thermodynamic parameters of the mixture. Based on this model, 2D numerical simulations of irregular multifront detonation wave in a stoichiometric methane/air mixture is performed. The calculated results of the detonation structure and cell width correspond very well to the available experimental data.