The effect of adding a nanoclay on the radical polymerization kinetics above the polymer's glass transition temperature was investigated through a simulation model. As an example, the polymerization of butyl methacrylate in the presence of organomodified nanomontmorillonite was studied. The mathematical model developed was based on the elementary chemical reactions taking place and appropriate species mass balance equations. To account for the effect of diffusion-controlled phenomena, appropriate theoretical equations were derived based on sound principles. The number of fitting parameters was kept to a minimum of four. It was found that the addition of the nanoclay mainly affects the initiator efficiency and the reaction diffusion term of the termination rate constant. Model predictions for monomer conversion were found to be in very good agreement with published experimental data. Moreover, the effect of the nanoclay on the evolution of the polymer average molecular weights during the reaction was discussed in detail.