We have extended the Continuous Configuration Boltzmann Biased (CCBB) Monte Carlo method to predicting the free energy and properties of multiple-chain condensed amorphous polymers. We illustrate the method by considering two independent polyethylene (PE) chains per cell (each with 100 monomers) using periodic boundary conditions (PBC). In CCBB-MC the free energy is calculated directly and simultaneously with pressure and other configuration dependent properties. The phase transition from polymer vapor to liquid is demonstrated by the density dependence of pressure and free energy. At low temperature, increasing the density causes the chains to change from globular to interpenetrating due to increased screening. At high density, increasing the temperature causes each chain to shrink since it becomes more flexible due to the screened interchain interactions. This leads to a dramatically increased population of short lengths in the end-to-end distance distribution.