Starting from a coarse-grained model, which includes hydrodynamic interactions, we numerically study kinetics of phase separation in a two-dimensional model of a polymer blend. In the absence of hydrodynamics, domains grow according to the Lifshitz-Slyozov power law without any evidence of pinning, regardless of the relative concentration of the blend components. In the presence of hydrodynamics, our results suggest that critical and off-critical mixtures are characterized by different growth laws. For the critical composition of the blend, hydrodynamic effects are found to be important during the entire growth process and this leads to a larger growth exponent. On the other hand, for off-critical blends, hydrodynamic effects are important only for blends with volume fraction not far away from the critical value. Even in these cases, domains show a faster growth only at intermediate times under the influence of hydrodynamics; subsequently the growth exponent crosses over to a Lifshitz-Slyozov value at late times. This gives strong support to the argument that the apparent pinning of domain growth observed in experiments on off-critical polymer blends may be due to this crossover phenomenon.