This article presents a method to predict the equilibrium composition and temperature of a reaction mixture in gas phase as found in the reaction furnace of the Claus Sulphur Recovery Unit (SRU). The proposed method is useful for process design as well as plant performance optimization. An expression for the Gibbs free energy of the system in terms of temperature is derived by using the Soave-Redlich-Kwong (SRK) equation of state. Temperature correlations for Gibbs free energy of formation of various components in the reaction mixture are obtained using standard heat capacity (Cp) temperature correlations. Initial assumption for the furnace outlet temperature is made to calculate the value for the Gibbs free energy of the system. Solver function minimizes this value by varying the composition of the outlet stream using mathematical optimization methods for non-linear systems. The outlet stream composition corresponding to this value is the equilibrium composition as the Gibbs free energy of a system reaches minimum at equilibrium. Application of Gibbs free energy minimization method using the Solver feature present in the Microsoft Excel spreadsheet program is found to predict effectively the equilibrium composition and temperature of the outlet streams of reaction mixtures in three plant case studies of Claus Sulphur Recovery Unit.