In this article the absorption process in a smooth-tube heat exchanger (GAX absorber) with a porous medium is analyzed and a countercurrent model for GAX absorber that includes an absorption process within the porous medium is proposed. The absorption process is separated into three regions, a vapor region without a porous medium (region I), a vapor region within a porous medium (region II), and a liquid region within a porous medium (region III). The effects of thermal conductivities and porosities of the porous medium on the absorption rate and the required tube length are investigated, and the Nusselt number within each region is also predicted. The results show that the absorption rate increases with an increase of the thermal conductivity and with a decrease in porosity of the porous medium. The optimum value of the ratio of thermal conductivities (kp/ k1)was found to be 5.0, and the optimum value of the porosity was found to be larger than 0.2.