From my visit last week, the Heat Experience is complimentary for all hotel guests. This agrees with their website under the "Access" section: https://www.ritzcarlton.com/en/hotel...ton-tokyo/spa/
If you are expecting that the Heat Experience is similar to a typical Japan onsen, then you might be somewhat disappointed. As 1200GS mentioned above, it is access to a sauna, steam room, cold bath, and hot tub. The hot tub is similar to a U.S. type hot tub (so somewhat small) except that you are nude. https://www.instagram.com/ritzcarlto.../p/C6bGRGPoIhC . With three or four people in the entire facility at one time, it felt crowded but maybe it was due to my modesty being naked in a hot tub.

FYI, there is maintenance scheduled between February 25 and March 16 so the men's area will be closed. The women's area will be closed between March 3 and March 11.