Crystal melt interfacial free energie ate important ingredients in predicting the nucleation barrier for polymers to crystallize. Experimental measurement of polymer crystal melt interfacial tensions is challenging, particularly for crystal facets other than the lamellar surfaces. We propose a simple way to obtain the interfacial free energy for any polymer crystal surface and melt using molecular dynamics simulation. We measure the force on a simulated nanoscale "plunger", that, restrains a melt from flowing into the gap between two crystals cleaved along the (100) plane. This gives the difference between the crystal vacuum and crystal melt interfacial free energies. Separately, the crystal vacuum interfacial free energy is obtained by measuring the force required to hold two crystals apart and integrating the force with respect to distance. We obtain the crystal melt interfacial free energy by subtracting the above values. Results from this method applied to n-alkanes can be compared to measurements of the homogeneous nucleation barrier.