We have developed a computer code (GEM-grain evolution model) to simulate the behavior of ice grains in a comet coma. The grains are assumed to be composed of water-ice with an admixture of dark material ("dirt"). An initial size distribution of grains is assumed to be ejected from the nucleus. The ejected mass is taken to be proportional to the rate of gas production by the nucleus. The efficiency for absorption and re-radiation of sunlight is computed from Mie scattering theory. The grain temperature and sublimation rate at a given heliocentric distance is then derived from energy balance considerations. The evolution of the grain size distribution is followed as a function of distance from the nucleus.