Is it permissible for one who experienced a wet dream to perform tayammum if he fears illness due to severe cold, even if hot water is available?

If it is possible to heat the water and bathe with it without harm, then it is obligatory. If not, then tayammum (dry ablution) is permissible. If you are unable to wash the entire body, then wash what you can and perform tayammum for the rest. There is an opinion held by some scholars that the prayer of a person in a state of major ritual impurity (junub) is valid if he suffices with ablution (wudu) when unable to perform a full ritual bath (ghusl), but this is an opinion contrary to the majority. It is obligatory to learn individual obligations (furood al-ayn), such as the rulings of purity and prayer.