A method for controlling a solution process for the polymerisation of olefins is disclosed. The method is based on an algorithm that calculates predicted operating parameters of the process, including calculation of the extent of reaction (ξ), as a function of the solution density (ρ), and then calculation of the whole composition of the solution. The algorithm resolves a system of equations comprising the equation of state in the PC-SAFT form, the Lewis-Mayo equation for monomer incorporation into a copolymer, and the stoichiometry equation. Predicted parameters are compared to target parameters to detect possible deviations and implement corresponding compensation in the feed.