The kinetics of the dimer-exchange mechanism in the surfactant-mediated Si/Ge epitaxial growth was investigated by using the density-functional theory calculations. Several dimer-exchange models were evaluated by constructing appropriate pathways. We found that two previously suggested models involved both pushing-out and rolling-over processes, giving rise to higher activation barriers than the surface diffusion barrier. We proposed a new pathway that exclusively involved the pushing-out process that gave a relatively lower activation barrier.