The in-situ formation of foamy oil has been found to be a crucial mechanism accounting for the better-than-expected production performance in heavy oil reservoirs under solution gas drive. To date, the physical laws dominating gas exsolution in foamy oil have not yet been well understood, while the generation of foamy oil is essentially arose from such an extremely complicated dynamic process. In this study, a novel and pragmatic technique has been proposed and validated to quantify the gas exsolution in bulk foamy oil under solution gas drive conditions by taking into account the gas bubble size distribution and the preferential mass transfer of each gas component. Experimentally, constant-composition expansion (CCE) tests with various constant-pressure decline rates are utilized to describe the gas exsolution behaviour of alkane solvent(s)-CO2-heavy oil systems under nonequilibrium conditions, during which not only pressure and volume are simultaneously monitored and measured, but also gas samples were respectively collected at the beginning and the end of experiments to perform compositional analysis. Theoretically, a mathematical model has been formulated to quantify gas exsolution process and the preferential mass transfer between of each gas component and liquid phase in alkane solvent(s)-CO2-heavy oil systems under nonequilibrium conditions. More specifically, quasi-equilibrium boundary conditions, real gas equation and Rayleigh distribution function are combined with classical equation of motion, continuity equation, and mass transfer equation to form a novel equation matrix for quantifying gas bubble growth in foamy oil. With consideration of gas bubble size distribution and preferential diffusion of each component in a gas mixture, the total number of gas bubbles and individual diffusion coefficient of each gas component are determined by minimizing the deviation between the measured volume of alkane solvent(s)-CO2-heavy oil systems and the calculated one. More importantly, the dynamic composition of gas phase and the amounts of both entrained gas and evolved gas also can be obtained simultaneously during the gas exsolution processes. Excellent agreements between the experimentally measured parameters (i.e., volume of foamy oil, composition of evolved gas, and volume of free gas) and the calculated ones have been respectively achieved. Compared with the individual diffusion coefficient for each component in a gas mixture determined under the traditional conditions, a relatively large value has been found during mass transfer processes in a supersaturated oleic phase. Also, pseudo-bubblepoint pressure and rate of gas exsolution is found to be two mechanisms dominating the volume-growth rate of the evolved gas.