We are presenting a new method to measure the chemical potential of polymer chains in computer simulations which has virtually no limitations in chain length. The method uses a thermodynamic integration over the excluded volume interaction of a ghost chain to determine its excess chemical potential. It is exemplified with a Monte Carlo simulation of the athermal bond flucutation model for various densities and the chain lengths N=20,50,80. Furthermore, the osmotic equation of state and the entropy density are determined.