Step 1: Identify the reaction.
Methane reacts with steam at high temperature in presence of nickel catalyst.
This reaction is called steam reforming of methane.
Step 2: Write the chemical equation.
\[
\ce{CH4 + H2O ->[Ni][1273K] CO + 3H2}
\]
Step 3: Identify the products.
The products are:
\[
CO
\]
and:
\[
H_2
\]
This mixture is also called synthesis gas or syngas.
Step 4: Final answer.
Therefore, methane reacts with steam to form:
\[
CO\text{ and }H_2.
\]