A combination of a molecular simulation method and the Monte Carlo method has been successfully utilized to calculate phase diagrams of model polyurethanes. In our model, the entropic contribution of the Flory-Huggins expression has been modified to incorporate the contribution arising from orientation of hard segments. The constraint associated with chain rigidity of hard segments has been explicitly considered. In addition, the interaction term has been modified to include the relative packing of hard segments. Phase diagrams of various MDI-PPG model polyurethanes have thus been predicted utilizing these modifications. The effects of soft- and hard-segment lengths have been considered and the actual degree of phase separation calculated. Our predictions have been compared to experimental values. Additionally, the contribution of hydrogen bonding to the miscibility behavior of hard and soft segments needs to be reevaluated.