Steam
Vapor
noun
Hot water's energy isn't the hot water itself, it's what comes out of it (when it's boiling, for example). This is the steam and vapor.
The ゆ part of the reading makes sense. The げ part for 気 is a bit of an exception, so here's a mnemonic to help with that:
No creature likes steam and vapor more than geckos (げ). Geckos live in hot places, so wherever you see steam, you'll be sure to see at least one gecko nearby, if you look hard enough.
ケトルから湯気が出てるよ。
Steam is coming out of the kettle.
湯気に気を付けてね。
Be careful of the steam.
美味しい寿司飯を作るには、しゃもじを使って、湯気が立つ炊きたての熱いごはんに酢を混ぜる必要があります。
To make good sushi rice, using a paddle, you must mix vinegar into steaming hot, freshly cooked rice.