You could buy NG futures and sell oil futures, bit there’s a catch: NG futures already have a large % increase relative to the current spot price baked in.
An alternative way to play NG is to own shares of APC (#msg-40563814), which gives you two ways to win. APC has large positions in the major US gas-shale regions and hence it will strongly benefit from a rising price of domestic gas. Moreover, APC has a high-impact exploration program all over the world, so it will benefit from rising hydrocarbon prices regardless of the spread between oil and NG.
APC and HES are my two favorite mid-cap energy companies. HES has a high-impact exploration program comparable to APC’s, but HES’ cash flow to support the exploration program comes from refining and marketing rather than NG production.