Hi @duongthang,

This construction is correct.
Check out this sentence from wsj.
Mr. Knight said he believed part of the reason that Google's new search algorithm demoted Ezinearticles.com articles is that much of the

Coming back to the sentence in question:
The reason that certain spicy foods, such as the Habanero pepper, make some people sweat is that these foods contain a chemical that stimulates the same nerves in the mouth and on the tongue as does a rise in temperature.

This sentence could have been re-written as follows too:
The reason certain spicy foods, such as the Habanero pepper, make some people sweat is that these foods contain a chemical that stimulates the same nerves in the mouth and on the tongue as does a rise in temperature.

The important thing to keep in mind is that we say - The reason...is THAT and not The reason...is because..
(I was actually surprised to see the latter redundant construction used in a few articles in newsweek and wsj.)