In this work, we test the recently developed generalized Redlich-Kwong/Peng-Robinson (RK-PR) three-parameter cubic equation of state for predicting thermodynamic properties of light, synthetic natural gas mixtures using the simple one-fluid mixing rule. The required binary interaction parameters (considered temperature-independent) come from correlation of literature vapor-liquid equilibrium data for the involved binary systems covering wide temperature and pressure ranges. Densities and phase envelopes of natural gas mixtures calculated with the equation of state are compared to highly accurate literature data to check the validity of the model. We also report comparisons with the predictions obtained from the Patel-Teja (PT) and the molecular-based, perturbed-chain SAFT (PC-SAFT) equations of state. RK-PR has good predictive ability for these systems considering its simplicity.