a small change is enough :)
if (i == 1)
ExcelApp.Cells[i, j] = dcCollection[j - 1].ToString();
ExcelApp.Cells[i+1, j] = dtMainSQLData.Rows[i - 1][j - 1].ToString();
changes:
1- 'else' removed.
2- rows started from 2 on excel sheet.
ps: use the debugger, the easiest way.