T-SQL - Projecting multiple records into one row for each key

A table can have multiple records for each key as below.

Actual Table:

Key         Value
----------- ---------
2149        805501   
2149        800936   
15385       800622   
18105       997057   
18105       999390   

Expected result:

Key         Value
----------- ---------
2149         805501,800936
15385       800622   
18105       997057,999390   

How can I solve this?

You should use FOR XML PATH:

Select Key, STUFF((SELECT ',' + VALUE
                   FROM Table AS T2 
                   WHERE T1.Key = T2.Key
                  FOR XML PATH('')), 1, 1, '') 
FROM Table T1
GROUP BY Key

UPDATE 1

based on your comments