The Seebeck coefficient study in a heavily nitrogen-doped n-type 4H-SiC epilayer in the direction perpendicular to c-axis is presented. The Seebeck coefficient steeply increases from 0.56 mV/K to 1.7 mV/K with decreasing temperature in the range 400-80 R. This behavior is explained by the phonon drag effect. An approach to the theoretical modeling of the phonon drag effect is discussed and simulation of the Seebeck coefficient temperature-dependence is displayed.