Experimental studies of the diffusion of small molecules in polymers typically report mutual diffusivities while simulation studies report tracer diffusivities. The Darken equation has been used to relate measured mutual diffusivities to calculated tracer diffusivities. The current study uses grand canonical molecular dynamics (GCMD) to simulate a gradient in chemical potential and calculate the mutual diffusivity in a polymer/penetrant system. The penetrants are modeled as hard spheres while the polymer is modeled as a collection of stationary hard chains. Penetrant mutual diffusivity is a function of two competing factors, a thermodynamic factor and a friction factor. An increase in the chemical potential of the penetrant tends to increase the diffusivity, the thermodynamic factor effect, while a corresponding increase in the number of particles within the system tends to decrease the diffusivity, the friction factor effect. Mutual diffusivity differs from penetrant tracer diffusivity by almost an order of magnitude. Mutual diffusivities do not obey the Darken relation for the simulation model used in this study.