from PyQt4 import QtCore, QtGui

try:
    _fromUtf8 = QtCore.QString.fromUtf8
except AttributeError:
    def _fromUtf8(s):
        return s

try:
    _encoding = QtGui.QApplication.UnicodeUTF8
    def _translate(context, text, disambig):
        return QtGui.QApplication.translate(context, text, disambig, _encoding)
except AttributeError:
    def _translate(context, text, disambig):
        return QtGui.QApplication.translate(context, text, disambig)

class Ui_Dialog(object):
    def setupUi(self, Dialog):
        Dialog.setObjectName(_fromUtf8("Dialog"))
        Dialog.resize(400, 300)
        self.gridLayout = QtGui.QGridLayout(Dialog)
        self.gridLayout.setObjectName(_fromUtf8("gridLayout"))
        self.textEdit = QtGui.QTextEdit(Dialog)
        self.textEdit.setObjectName(_fromUtf8("textEdit"))
        self.gridLayout.addWidget(self.textEdit, 0, 0, 1, 1)

        self.retranslateUi(Dialog)
        QtCore.QMetaObject.connectSlotsByName(Dialog)

    def retranslateUi(self, Dialog):
        Dialog.setWindowTitle(_translate("Dialog", "Dialog", None))

import sys

from qtdesigner.dialog_reacciones_rsa import *

class Window(QtGui.QMainWindow):

    def __init__(self, parent=None):

        QtGui.QMainWindow.__init__(self, parent)
        self.ui = Ui_Dialog()
        self.ui.setupUi(self)

        self.ui.textEdit.setText('HolanAdios')


if __name__ == '__main__':

    app = QtGui.QApplication(sys.argv)
    myapp = Window()
    myapp.show()
    sys.exit(app.exec_())

...
class Window(QtGui.QDialog):

        def __init__(self, parent=None):

            QtGui.QDialog.__init__(self, parent)
...