The modeling of high-pressure combustion presents a continuing challenge in the context of chemical kinetics and mixture transport properties. This is mainly due to a lack of experimental data at high pressures. Consequently, there is limited confidence in the accuracy of simulations above moderate pressures. In this paper real gas thermodynamics is included in reacting flow simulations, e. g., by adopting the Redlich-Kwong equation of state, modifying the expression of the equilibrium reaction rate constants that are used to determine reverse reaction rates in a detailed reaction mechanism, and considering high-pressure effects on the enthalpy and specific heat. These real gas effects are characterized through simulations of freely propagating flames in methane/ oxygen/inert mixtures at pressures up to 150 atm. Our results indicate that the laminar flame speed decreases more rapidly with increasing pressure when the real gas formulation is employed than for an ideal gas, particularly for pressures greater than 40 atm. The differences can be ascribed to high-pressure effects on the mixture specific heat. The incorporation of real gas thermodynamics improves the agreement of predictions with measurements.