Extended-basis-set calculations with electron correlation have been carried out for the reactant and transition state properties of the gas-phase S(N)2 reactions Cl- + CH3Br --> CH3Cl + Br-, Cl- + CH3I --> CH3Cl + I-, and Br- CH3I --> CH3Br + I-. The resulting force fields are used for canonical unified statistical theory calculations of the rate constants of these reactions and their perdeuterated analogs. Kinetic isotope effects (KIEs) and their temperature dependences have been calculated and analyzed, and they are compared to available experimental data. We find only small deviations of the generalized transition state theory results from conventional transition state theory, and we obtain reasonable agreement (similar to 10%) with experimental results for the KIEs of the first two reactions, but the KIEs that we predict for the third reaction are up to 26% higher than experiment.