Highly branched alkanes exhibit enhanced free volume relative to their straight chain analogues, leading to the increased solubility of sparingly soluble gases, such as N-2, as well as lower hydrocarbon-gas interfacial tension (IFT) values. In this study, high-pressure, high-temperature IFT data are reported for two C16 isomers, hexadecane (HXD) and heptamethylnonane (HMN), with N-2 at temperatures from similar to 298 to 573 K and pressures up to 100 MPa. The IFT data are modeled with density gradient theory (DGT) in conjunction with the perturbed-chain, statistical associating fluid theory equation of state (EoS) with pure component parameters calculated with three different group contribution (GC) methods. One GC method (B-GC) is developed from a database of high-pressure density data, and the other two GC methods (S-GC and T-GC) are developed from a large database of pure component vapor pressure and saturated liquid density data. DGT calculations incorporating the B-GC method reasonably represent the IFT for both HXD + N-2 and HMN + N-2 at low temperatures but result in significant deviations from experimental IFT values at high temperatures. The S-GC method provides improved IFT predictions relative to the B-GC method at high temperatures, but S-GC predictions are inferior to those obtained using the T-GC method. The superior performance of the T-GC method is attributed to the use of second-order GC parameters and to the ability of this method to more correctly predict EoS parameters for both normal and branched alkanes.