The passage of steam over hot coke. The mixture of CO and $\mathrm{H}_2$ thus produced is known as water gas or synthesis gas.
$\mathrm{C}(\mathrm{~s})+\mathrm{H}_2 \mathrm{O}(\mathrm{~g}) \xrightarrow{473-/ 273 \mathrm{~K}} \mathrm{CO}(\mathrm{~g})+\mathrm{H}_2(\mathrm{~g})$
water gas