The heat transfer and fluid flow are determined for fully deveIoped laminar flow in a vertical channel. The effects of natural and forced convection and thermal radiation are included in the analysis. The spectral variation of the radiation properties are included through the use of the total band absorp~ tance. The analysis is general and specific numerical results have been obtained for the 15 micron band of carbon dioxide. A simplified approach for large path is also presented which is in excellent agreement with the more exact results.