The accurate prediction of thermodynamic properties of natural gas systems is of interest for gas industry. Compressibility factors are used in energy and flow metering. It is also used in calculations of gas pressure gradient in tubing and pipelines. When large volumes of gas are traded between producers, distributors, and consumers, error in the estimation of the amount of gas involved are of real economic significance. In gas condensate reservoirs, well-productivity often declines rapidly when pressure drops below the dew point pressure near-wellbore. Therefore, it is very important to accurately determine the dew point pressure. The pressure and temperature of most natural gas mixtures can be found up to 150 MPa and 500 K, respectively (Nasrifar and Boland, 2006). At these conditions, methane, ethane, and nitrogen are almost always supercritical while other hydrocarbons are subcritical. Thus, the equation of state of natural gas mixture must be accurate at supercritical and subcritical behavior of methane and heavy hydrocarbons, respectively.