A methodology for including the effects of nonidealities, such as confinement in a porous solid or solvation, into the calculation of bimolecular reaction rate constants is presented. The method combines the transition-state theory formalism with the Reactive Monte Carlo simulation method. The approach is computationally efficient and accurate, within the approximations imposed by transition-state theory and the intermolecular potentials. Several applications of the method are presented for the decomposition reaction, 2HI-->H-2+I-2, including effects due to confinement within carbon micropores and due to inert solvents. The method can be readily extended to other chemical reaction rate calculations in which the structure and the activation energy of the transition state is known a priori.