Within the framework of reduced-dimensionality quantum scattering theory, we employ Bowman's adiabatic rotation approximation to describe reactive systems that have symmetric-top geometries during the entire collision process. The results are compared with the approach of shifting the total energy by a characteristic rotational energy. Initial state-selected and total thermal rate constants have been computed for the complex-forming gas-phase reaction Cl- + CH3Cl' -> ClCH3 + Cl'(-). At room temperature, we find a significant contribution from energetically high vibrational modes. The dependence of the cross-sections on the different angular momenta is analyzed in detail, and high total angular momenta are found to be of considerable importance. The influence of adiabatic azimuthal rotation on the rate constants turns out to be small compared to other effects. In addition, we use a new model to account for the asymmetric modes not explicitly contained in the scattering calculations. The difference to the only available experimental value confirms our conclusion that the Cl-C-Cl' bending modes are of major importance for this reaction.