A comprehensive theory for the Soret effect (also called thermal diffusion) is presented which incorporates both the thermodynamic contribution from selective attraction/repulsion and the kinetic contribution from selective collision interaction between the components. The new theory is an extension of a theory presented earlier in which the thermodynamic contribution only was modeled. The single assumption of the theory is that the Soret effect in the steady state is the macroscopic state accomplished by a maximum number of microstates with respect to the ideal gas state. As a result, the Soret effect in a multicomponent mixture can be calculated by using input from an equation-of-state of the mixture and kinetic gas theory without the use of matching parameters. The theory is not limited to systems with a small temperature difference and/or a small concentration difference. The methodology of the new theory can be used to model other cross-effects in irreversible thermodynamics. A test of the theory against the measured Soret effect in 18 mixtures shows agreement within a factor of 2 over four decades. Closer agreement cannot be expected since it appears that the calculation of the Soret effect is extremely sensitive to the accuracy of input from the equation-of-state. The present equations-of-state, even those that are calibrated for use in the chemical and petroleum industry, require modification for the calculation of the Soret effect, because of a higher demand in accuracy. In addition, it is also important to examine which frame of reference (center-of-volume or center-of-mass) applies to a particular measurement or practical application, because the frame of reference determines which mathematical expression for the Soret effect must be used.