For the mechanical calculation we use PVElite, and you can also use COMPRESS.
According to my opinion it is better to purchase different program for mechanical and thermal calculation.
For thermal calculation HTRI is very suitable and is has interface with PVElite 2010.
But if you are not have to make a lot of heat exchangers HTRI is quite expensive according to my opinion.
We use for thermal calculations a fortran program which utilizes well known methods you can find them at most heat exchangers books ( heat exchanger design handbook, Perry - no traditional hex book,but a good one source-). Personal I based to wolverine heat exhanger handbook you can find it at the relative site.With little effort you can make a program and solve the thermal design problem with very good accuracy.