A computer simulation has been obtained of the atom recombination reaction in which the recombination energy is removed by a third-body. The equations of classical dynamics have been solved for two iodine atoms and six inert gas atoms (He, Ar or Xe) confined to a spherical vessel by a wall potential which is a spatial average of a spherical shell of inert gas atoms. Reactions at a given temperature and concentration are simulated by varying the initial momenta of the atoms and the volume of the sphere. A computer run of 100 trajectories for each physical situation gives the average time for recombination, and from this a macroscopic rate constant has been calculated that agrees well with the experimental result. The model reproduces all the characteristic kinetic mechanisms that have traditionally been used to interpret atom recombination. However, at high inert gas concentrations the steady-state approximation is shown to fail as many of the important intermediate reactions do not reach equilibrium. In this high concentration region the fall-off of the recombination rate constant is discussed in terms of a diffusion controlled mechanism.