A finite element model is used to predict the effect of composition on long spacing (L) in blends of crystalline and noncrystalline polymers. In this model, the diffusive transport of noncrystallizable moieties away from the crystals from which they are excluded is tracked. The effect of the local concentration of noncrystallizable material near the growing interface on the kinetics of crystallization is modeled. Parameters in the model are the Peclet number (Pe), the linear crystallinity (phi(c)), and the overall concentration of noncrystallizable molecules (C.). The overall crystallization rate (T) depends on these three parameters. T is found to exhibit a maximum (T-max) with respect to phi(c) at given C-o and Pe. It is shown that, over a considerable range of Pe and C-o, T-max exhibits either a nearly constant low value nor a nearly constant higher value. It is suggested that T-max is the operating condition for such crystallization. From this hypothesis, expected values of long spacing are predicted. The dependence of L on C-o is found to lie on either of two branches, depending on the value of Pe. Large Pe shows a small effect of C-o on L, while high Pe exhibits a relatively large effect. These results agree with observations on blend systems.