The direct simulation Monte Carlo (DSMC) method is a general particle based technique for computation of nonequilibrium gas flows. A number of chemistry models have been developed for DSMC that make it possible to analyze high-energy flows including the aerothermodynamics of planetary entry vehicles. In the present work, an important limitation of existing DSMC models for simulating the backward (exothermic) direction of chemical reactions is identified. A new model is proposed to address the problem. Results obtained with the new chemistry model demonstrate the ability to accurately predict backward rate coefficients and chemical composition under equilibrium conditions. Application of the new model to a high-velocity, nonequilibrium flow condition indicates differences in species concentrations in comparison to the existing, physically inaccurate approach for modeling the backward reaction steps.