Methane adsorption at Ca decorated single wall carbon nanotubes has been studied by ab initio total energy calculations based on the density functional theory. The adsorption configurations have been studied by using various exchange-correlation energy functionals also including two possible long-range interaction correction schemes. Our calculations show that methane adsorption at Ca decorated carbon nanotubes is markedly enhanced when impurity atoms are considered as individual adsorption sites. We demonstrate that up to six CH4 molecules can bind at a single Ca impurity at room temperature. The phenomenon responsible for the measured adsorption energy is recognized as a Kubas-type interaction that involves the orbital overlap between the Ca d state and the methane sigma molecular orbitals. The adsorption values obtained with the different energy functionals used are discussed showing that local density approximation, often employed in the recent literature for similar adsorption problems, is affected by severe limitations when orbital overlap and physisorption co-exist. Lastly the well-known problem of Ca clustering is studied and discussed showing that, different from the hydrogen case, it does not induce any molecular dissociation.