Phase behavior calculation of hydrocarbon mixtures and gas condensate systems in the near-critical and retrograde regions is of a fundamental nature. Current approaches using an equation of state cannot be used reliably to calculate the phase behavior of gas condensates in the near-critical region, and the use of complicated mixing rules may not yield reliable results. In this paper, we propose to modify the binary interaction coefficients. These coefficients, contrary to the current assumption, may not be constant (even at a given temperature), but are a function of the distance to the critical point of the system. When this variation is accounted for, significant improvement in the description of the phase behavior around the critical point can be obtained.