A method of operating a dedicated-EGR engine includes providing a rich air-fuel mixture to a dedicated cylinder; combusting the rich air-fuel mixture in the dedicated cylinder; modeling the combustion of the rich air-fuel mixture in the dedicated cylinder; estimating the composition of the combustion products in the dedicated cylinder based on interpolation of chemical reaction models of stoichiometric and rich combustion. The method further includes mixing the combustion products from the dedicated cylinder with air to produce an intake mixture; estimating a mass fraction of reformate and a mass fraction of burned gas in the intake mixture; providing the intake mixture to the intake ports of all of the cylinders of the dedicated-EGR engine; combusting an air-fuel mixture in a non-dedicated cylinder of the engine; and controlling an engine control parameter based on the estimated mass fractions of reformate and burned gas in the intake mixture.