Converting Multiple Rows into a CSV String (Set Based Method)

Prod_Code
---------
2001
2002
2003
2004

I need to end up with '2001,2002,2003,2004'.

Query:
=======
declare @str varchar(8000)
select @str = COALESCE(@str + ', ','') + convert(varchar(4),Col_name)
from Table_Name

select @str