A cubic equation of state, with simple non-density-dependent mixing rules, is used to represent the phase behaviour of multicomponent mixtures containing hydrocarbons up to n-octane, nitrogen, carbon dioxide, hydrogen sulphide, water and methanol. The reliability of the model is tested against diverse experimental VL and VLL equilibrium data of multicomponent systems. The proposed correlation can be applied for efficient design and operation of transfer lines and processing units. The practical value of the theoretically correct concept of density dependence is tested on a density-dependent version of the above mixing rules. It is demonstrated that while both mixing rules produce very similar results when only one polar component is present, the density-dependent mixing rules may be inferior for mixtures containing both water and methanol, owing to an inherent deficiency. It is also demonstrated that an invariant version of the proposed non-density-dependent mixing rule may not lead to indisputably improved predictions for the studied systems.