A nanocomposite of multiwalled carbon nanotubes (MIATCNTs) and the metal-organic framework MOF-199, denoted as CNT@MOF-199, has been synthesized. It was prepared by the incorporation of multiwalled carbon nanotubes into MOF-199, which exhibits an increased micropore volume, enhanced gas storage capacity, and improved stability compared to MOF-199. The adsorption capacities of gas molecules in MOF-199 and CNT@MOF-199 were found to decrease with increasing temperature, indicating the exothermic nature of the adsorption process. Novel functionalized adsorbents were synthesized by impregnation of various amounts (10, 20, and 30 wt %) of piperazine (PZ) on synthetic CNT@MOF-199. Although the surface area, pore size, and pore volume decreased after modification, CNT@MOF-199/PZ exhibited a higher adsorption capacity and selectivity than CNT@MOF-199 at the pressure under study because of the existence of a great affinity between CO2 molecules as a Lewis acid and the basic amine sites on CNT@MOF-199/PZ. The physicochemical properties of the adsorbents were characterized by N-2 adsorption/desorption isotherms, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). The highest selectivity was observed at 298 K for CNT@MOF-199/30PZ.