An effective quasi-one-component model for homogeneous vapour-liquid nucleation of non-ideal binary mixtures in the regime of retrograde condensation is proposed. A transformation from the original binary to an effective unary system is accompanied by the appearance of an extra dependence of properties on cluster size. For the Gibbs free energy of cluster formation a generalized Fisher droplet model is formulated and applied at the effective saturation point within the framework of kinetic approach. The theory contains no adjustable parameters. The mixture w-nonane/methane is studied at conditions corresponding to the retrograde condensation (pressures > 10 bar). Calculated nucleation rates agree well with the recent experimental data whereas predictions of the classical binary-nucleation theory are 3-4 orders of magnitude lower.