How do you separate mixture of carbon dioxide and steam?

David's suggestion has some flaws. If the mixture of water vapor and CO2 is cooled so that the water condenses, then the CO2 will dissolve in the water as the solution cools. That is just the opposite of what you want. In fact, boiling water is how we get rid of (or at least minimize) the CO2 which may be dissolved in water. That works because the solubility of gases decreases as the temperature of the solution increases.

To separate the mixture of water vapor and CO2, a process called CO2 "scrubbing", pass the mixture over calcium oxide, CaO. The CO2 will combine with it to make solid calcium carbonate, CaCO3. Water vapor will react with CaO to make solid calcium hydroxide, Ca(OH)2.

CaO(s) + CO2(g) --> CaCO3(s)
CaO(s) + H2O(g) --> Ca(OH)2(s)

The mixture of CaCO3 and Ca(OH)2 can be separated by adding it to liquid water. Calcium hydroxide is somewhat soluble in water and calcium carbonate is not.
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cool to a temp < 100C and the steam will condense to liquid water CO2 stays a gas