In some applications of rarefied gas dynamics, including flows in aerospace/space engineering, film deposition technique in semiconductor and micro-reactor in medical science, chemical reactions play an important role and can not be ignored in these simulations. To understand effects of chemical reactions in these fields by experiment is very expensive and impractical. Thus, an appropriate simulation method is worthy to develop. A particle method named the direct simulation of Monte Carlo method (DSMC) is used to simulate these complicated flows and chemical-reaction module can be incorporated properly in the molecular level. In the current study, a chemical-reaction module, including dissociation, exchange and recombination for air species, is developed and implemented into a previously developed general-purpose parallel DSMC code (PDSC). The total collision energy model (TCE) is used to calculate the reaction probabilities for dissociation and exchange reactions. A three-body collision model, proposed by Boyd, is utilized to treat the recombination reaction. The reaction probability of each chemical reaction and the degree of dissociation of a single-cell are used to verify the chemical-reaction module by comparing with theoretical data. Then three different types of hypersonic re-entry vehicles, including sphere, blunt-body and Apollo, are simulated to understand the effects of chemical reactions.