The Combining of Carbon Dioxide and Hydrogen
I believe you are referring to the Sabatier process. $$\ce{CO2 + 4H2 -&gt; CH4 + 2H2O}$$

The reaction uses a $\ce{Ni}$ catalyst and is done at $300-400\,^{\circ}{\rm C}$ and high pressures. The reaction is exothermic, $\Delta H = −165.0~\mathrm{kJ\,mol}^{-1}$, but requires initial heating in order to start the reaction off due to the activation energy.

Read more on Wikipedia.