A model to treat Ge segregation with simultaneous growth and exchange during Si/SiGe layer growth by molecular beam epitaxy is described. Within this three layer formalism, the segregating layers were treated in two limiting cases, a solid surface model in which no surface diffusion occurs, and a fluid surface model, in which surface diffusion is very fast. Simultaneous treatment of exchange and growth within the fluid surface model was the only one of the two that allowed the accumulation of Ge in the top two layers to significantly exceed that of the bulk in surface alloys, in agreement with experimental observations.