Partial Pressures

If the keg is filled with more than one type of gas molecule (N2 and CO2 for example) each gas acts independently. If there were enough CO2 molecules in a keg to generate 15 psi of CO2 and you added enough N2 to bring the total pressure to 25 psi you still have 15 psi worth of CO2 molecules hitting the surface and entering solution. If the CO2 was in equilibrium before adding the N2 it will stay at equilibrium after adding the N2: Adding enough N2 to take the total pressure to 1000 psi will not change the number of CO2 molecules or the force with which they strike the surface or the amount of CO2 dissolved in the beer.